As a preferable example of liquefied gas fuel, dimethyl ether (DME) is used as the fuel of an internal combustion engine.
Since the heat generation rate of the liquid DME is lower than that light oil, it is necessary to increase the injection quantity of DME more than that the light oil. Thus, in a case that the liquid DME is injected, the injection period and combustion period are extended. As a result, especially, when an engine speed is high, the performance of the DME may be less than the light oil.
By heating the liquid DME, the liquid DME is brought into the supercritical state. The mixture of the supercritical DME and the air is promoted. Thus, while the injection period is long, the combustion period can be shortened.
Furthermore, by injecting the DME at the supercritical state, the injection rate and a heat generating rate become homothetic. Thus, the combustion characteristics can be controlled by the injection rate.
Since the combustion characteristics can be controlled by the injection rate, it is unnecessary to perform a multi-stage injection. Moreover, by injecting the DME at the supercritical state, it can be restricted that the fuel pressure becomes excessively high, as compared with a case where the liquid phase DME is injected.
(Issue 1)
The supercritical fluid can cause the same flow as gas. Thus, when the DME is injected from the nozzle hole at the supersonic velocity, the mixture of the DME and the air can be promoted.
When the lift amount of a needle is small (small lift), the valve opening portion functions as a throttle, so that flow velocity of the fuel reaches acoustic velocity at the valve opening portion.
In a case that a nozzle seat surface and a tip end of the needle are conic surfaces, the clearance between the nozzle seat surface and the tip end of the needle becomes gradually large in a fuel flow direction. In such a shape, the fuel flow of acoustic velocity will be choked and the fuel amount is decreased. Especially, when the lift amount of the needle is very small, the velocity variation (pressure variation) is generated between upstream and downstream of the valve opening portion, which causes a large energy loss.
The flow velocity of the fuel passed through between the nozzle seat and the tip end of the needle is further accelerated in the sack chamber. Then, the fuel flows into the nozzle hole. Further, the fuel pressure is recovered in the nozzle hole and is accelerated. However, the fuel velocity does not reach acoustic velocity.
That is, when the needle lift amount is small, the flow velocity does not reach the acoustic velocity.
(Issue 2)
When the needle lift amount is large, the valve opening portion and the sack chamber function as a fuel passage so that the fuel flow is not attenuated.
However, in a case that the nozzle hole is a straight hole, the fuel flow velocity reaches the acoustic velocity at an outlet of the nozzle hole. In other words, the fuel cannot be injected from the nozzle hole at the supersonic velocity.
(Issue 3)
In order to avoid the above Issue 2, it is considered that the nozzle hole is formed as Laval shape, which is not well known technology.
However, in the case that the needle lift amount is small, the fuel flow velocity is attenuated. Thus, even if the Laval shape is employed, the fuel flow velocity cannot reach the supersonic velocity.
JP-2012-145048A shows a fuel injection valve having a nozzle hole that is comprised of two tapered surfaces.